They arrive in California each winter, an undulating ribbon of orange and black. There, migrating western monarch butterflies nestle among the state’s coastal forests, traveling from as far away as Idaho and Utah only to return home in the spring.

This year, though, the monarchs’ flight seems more perilous than ever. The Xerces Society for Invertebrate Conservation, a nonprofit group that conducts a yearly census of the western monarch, said the population reached historic lows in 2018, an estimated 86 percent decline from the previous year.

That in itself would be troubling news. But, combined with a 97 percent decline in the total population since the 1980s, this year’s count is “potentially catastrophic,” according to the biologist Emma Pelton.

“We think this is a huge wake-up call,” said Ms. Pelton, who oversees the survey and lives in Portland, Ore.

The society has preliminary counts from 97 sites, most of them along California’s coast, representing an area that traditionally accounts for nearly 77 percent of the state’s winter monarch population. In 2017, the sites hosted about 148,000 monarchs. But in 2018, that dropped to an estimated 20,456 monarchs, with large numbers of them counted in Pismo Beach, Big Sur and Pacific Grove.

In November volunteers fan out across California’s coastal cities to find and count the monarch population. Ms. Pelton said the total count could be higher once final numbers from the census arrive next week.

Monarchs in the western part of the United States migrate for the winter to California, where they gather mostly among fragrant eucalyptus trees, which provide hospitable living conditions.

Monarchs from the eastern part of the United States, by contrast, winter in Mexico. Ms. Pelton said the count of eastern monarchs had not been released.

Ms. Pelton warns that if nothing is done to preserve the western monarchs and their habitat, the butterflies could face extinction. In a 2017 study, scientists estimated that the monarch butterfly population in western North America had a 72 percent chance of becoming near extinct in 20 years if the monarch population trend was not reversed.

One of the study’s researchers, Cheryl Schultz, an associate professor at Washington State University Vancouver, said at the time that an estimated 10 million monarchs spent the winter in coastal California in the 1980s.

Butterflies are important because they quickly respond to ecological changes and serve as a warning about an ecosystem’s health, Ms. Pelton said. They pollinate flowers, too.

Monarchs require milkweed, a herbaceous plant that grows throughout the United States and Mexico, for breeding and migration. Acreage of milkweed, though, has been declining in recent years because of pesticide use and urban development, Ms. Pelton said.

“A lawn does not provide a home for a butterfly,” she said. “It doesn’t help to raise them in your house, either.”

Harsher than usual weather, too, has threatened the monarch’s existence. From 2011 to 2017, California had one of its worst droughts on record, which led to ecological devastation among fishing communities and forested towns.

In 2016, for example, the United States Forest Service estimated that 62 million trees died in the state. Last year the state experienced the deadliest wildfire season in its history, with residents affected from Redding to Los Angeles.

Ms. Pelton said the trend could reverse if citizens and governments act now. Gardeners, for one, can plant milkweed to support the surviving monarchs. And towns could help local habitats thrive by planting new trees now so that in 20 years, generations of monarchs have new places to winter.

“We don’t think it is too late to act,” she said. “But everyone needs to step up their effort.”

A version of this article appears in print on , on Page D2 of the New York edition with the headline: Perilous Journey: In California, Alarm as Population of Monarch Butterflies Plummets. Order Reprints | Today’s Paper | Subscribe